1. Field of the Invention
The present invention relates to anode active materials, methods of preparing the same, and anode and lithium batteries containing the anode active materials. More particularly, the invention is directed to an anode active material having pores for buffering volume changes during charging and discharging. The invention is also directed to a lithium battery having a long cycle life, which lithium battery employs the anode active material.
2. Description of the Related Art
Non-aqueous electrolyte secondary batteries, which include anodes comprising lithium compounds, exhibit high voltage and high energy density, and have therefore been widely researched. Lithium metal has been studied as an anode material because of its high capacity. However, when metallic lithium is used as an anode material, lithium dendrites are deposited onto the surface of the metallic lithium during charging. The lithium dendrites reduce the charge/discharge efficiency of the battery, and can cause short-circuits. Also, the risk of explosion and high sensitivity to heat and shock caused by the instability and high reactivity of metallic lithium has prevented the commercialization of metallic lithium anode batteries.
Carbon-based anodes have been used to address the problems of lithium anodes described above. Lithium ions present in an electrolyte are intercalated and deintercalated between crystal facets of the carbon-based anode, resulting in the occurrence of oxidation and reduction reactions. A battery including a carbon-based anode is referred to as a rocking chair battery.
Carbon-based anodes have addressed various problems caused by lithium metal, and have become popular. However, there is a need for lithium secondary batteries with high capacity in order to allow for minimization, reductions in weight and increases in power of portable electronic devices. Lithium batteries containing carbon-based anodes have low capacity due to the porous structure of carbon. For example, even for graphite (which is the carbon structure with the highest crystallinity) the theoretical capacity of a LiC6 composition is about 372 mAh/g. This is less than 10% of the theoretical capacity of lithium metal, which is about 3860 mAh/g. Therefore, despite the existing problems with metallic lithium, much research has been actively performed to improve the capacity of batteries by introducing metals such as lithium into the anodes.
It is known that Li and alloys such as Li—Al, Li—Pb, Li—Sn and Li—Si provide higher electrical capacities than carbon-based materials. However, when such alloys or metals are used by themselves, the deposition of lithium dendrites occurs. Therefore, use of a suitable mixture of such alloys or metals and carbon-based materials has been researched to provide high electrical capacity while also avoiding problems such as short circuits.
However, in such a mixture of metal materials and carbon-based materials, the volume expansion coefficient during oxidation and reduction of the carbon-based materials is different from that of the metal materials, and the metal materials can react with the electrolyte. When charging the anode material, lithium ions are introduced into the anode material. When this happens, the anode expands and also becomes more dense. On discharging, the lithium ions leave the anode, and the volume of the anode decreases. At this time, if the anode contracts, there remain voids in the anode that are not electrically connected due to the difference between the expansion coefficients of the carbon-based materials and the metal materials. Due to the electrically insulated voids, the movement of electrons is not effective and the efficiency of the battery is decreased. Also, a reaction between the metal materials and the electrolyte during the charging and discharging can decrease the lifetime of the electrolyte, thereby decreasing the lifetime and efficiency of the battery.
To overcome these problems, electrodes have been prepared by adding polymer additives for providing elasticity and pores to the active material. The polymer additive is simply mixed with the active material in the process of manufacturing the electrode, and provides elasticity and pores to the active material to enhance the cycle properties of the battery. However, since the additive is not adhered to the active material, electrical insulation may occur when the elasticity of the additive decreases due to long-term use.
An active material including two types of graphite having different surface densities has also been prepared. The energy density of the active material is increased by reducing pores between active material particles by mixing ball-shaped particles and needle-shaped particles. Additionally, an anode active material having spherical graphite and plate graphite has been prepared. An electrolyte can easily impregnate such an anode active material, and thus, the electrical capacity of the battery can be increased. However, only active materials, such as graphite, that do not show large volume changes during charging and discharging are suitable for such anode active materials. Thus active materials that show large volume changes during charging and discharging are not suitable.
Accordingly, a need exists for a more practical anode active material having excellent charge and discharge properties, a long lifetime and high efficiency.